situation in the first half of 1956 became complex as a result of the Russian explosion in November 1955. This test was announced as being “in the megaton class” and the associated fresh fission products were observed some time later in the lower stratosphere above the U. K. Two other tests of unspecified magnitude were announced in the spring of 1956. An upper limit can be derived for the Sr®® contribution of these three tests to the amount deposited at Milford Haven between January and May 1956 by attributing all the deposited Sr® to the test of November 1955. With this assumption, the associated Sr*’ has been computed and subtracted from the observed amount in each sampling period, giving the dotted curve in Fig. 3 which must represent, in the main, a lower limit for the amount of Sr® still attributable to the 1954 tests. After May 1956 it is not possible to fix the date of origin of the observed Sr® but, as is shown in Appendix 2, the wo RAIN WATER oe 1954 J F M A M 1955 J | 4 S 1 1 Oo N Sr9° IN STRATOSPHERIC AIR (RELATIVE SCALE) > Sr®° IN RAIN (ueC/LITER) data still point to the fact that the sr” deposited is primarily of stratospheric origin. 4, Fig. 4— Correlation between Sr** concentrations in rain water and in the lower stratosphere. An interesting result, supporting the stratospheric origin of the Sr®’ deposited during the period July 1954 to October 1955, shows a marked correlation between the concentration of Sr in rain water and that in the lower stratosphere. The Sr®’ concentration in the stratosphere was computed from the gross radioactivity collected, since during this period the lower strato- sphere contained fission products from the 1954 Eniwetok tests only. The correlation is demonstrated in Fig. 4 where the stratospheric content is plotted on a scale in which the mean concentrations at both 13,400 metres and 14,600 metres have been expressed relative to the values found at these heights in July 1954. Stratospheric samples collected at later dates will be ex- amined by the gamma spectrometer to determine if the correlation is maintained. Figures for the amount of Sr®? deposited at the other U. K. stations are given in the upper part of Table 3. The differences between the amounts deposited at the stations over common periods reflect different rainfall rates rather than differences in the specific contents of the rain. In the following table, the over-all specific activities of the rain water at the various sites have been expressed as percentages of that at Milford Haven over the common period of sampling. Rainfall rates have been similarly expressed: 23€